Complete kinetic model and process reengineering of glyoxal oxidation by nitric acid in a capillary microreactor†

The oxidation of glyoxal by nitric acid to glyoxylic acid is a complex process with parallel and consecutive side reactions. The complete reaction kinetics has not been thoroughly reported before. In this work, a continuous flow microreactor system, consisting of micromixers, preheating capillary loops, a capillary microreactor and quenching device, is designed to achieve oxidation under homogeneous conditions. A complete kinetic model is established and all kinetic parameters are obtained. The effects of the molar ratio of nitric acid to glyoxal, reaction temperature and concentration of nitric acid on the reaction are investigated systematically. Based on the process reengineering of existing devices, two schemes of segmented feeding (nitric acid in several segments) and recirculating feeding (incompletely reacted material is returned to the reactor for reaction) are proposed. Finally, the optimal reaction conditions are determined. At 68 °C (the initial molar ratio of nitric acid to glyoxal was 1.26, with a final molar ratio of 1.4 after segmented feeding at once, the molar ratio of sodium nitrite to glyoxal is 0.15, and the mass concentration of nitric acid is 35%), the yield of glyoxal acid is 89.2% and the selectivity is 95.9%. This work refines the kinetic data for the oxidation reaction of glyoxal nitrate. It is of theoretical importance for optimising reaction performance (temperature and residence time regulation strategies) and reactor design.